FRRS1L gene replacement ameliorates disease phenotypes in the mouse model of developmental and epileptic...

Gene therapy offers unique therapeutic potential for treating rare genetic disorders such as DEE37, a severe Developmental and Epileptic Encephalopathy caused by biallelic loss-of-function mutations in the Ferric Chelate Reductase 1 Like (FRRS1L) gene, presenting with seizures, developmental delay, dyskinesia, ataxia and progressive, eventually debilitating cognitive and motor impairments. There is currently no treatment for DEE37. FRRS1L encodes an auxiliary subunit essential for the tetrameric assembly of α-amino-3 hydroxy-5-methyl-4 isoxazolepropionic acid receptors (AMPARs) and their recruitment to synaptic membranes. FRRS1L is expressed brain-wide, with high levels in the cerebellum. Here, we developed an adeno-associated virus 9 (AAV9) gene replacement vector expressing human FRRS1L driven by the JeT promoter. Administering the virus intrathecally in two doses (5 × 10¹⁰ and 3 × 10¹¹ vg) to Frrs1l-knockout mice, we evaluated safety and efficacy in multiple phenotypic traits shared with affected patients. We obtained robust, near-normal, FRRS1L expression in the cerebellum and low levels of expression elsewhere in the brain, which were associated with corresponding levels of increased synaptic AMPAR abundance. Importantly, our phenotypic assessments revealed dose-dependent improvement trends, or rescue, across multiple brain structural, electrocorticographic, and behavioral domains. Our results indicate that cerebrospinal fluid-directed virally mediated FRRS1L gene replacement partially restores synaptic AMPAR and ameliorates disease-relevant phenotypes, supporting gene therapy as a promising therapeutic strategy for children suffering from this catastrophic, FRRS1L-related, 37th form of DEE.